(1) Field of the Art
The present invention relates to an optical resin having a high refractive index, and a diethylenically unsaturated monomer which gives this optical resin. More particularly, the present invention relates to an optical resin of a light weight, having an excellent heat resistance, an excellent optical uniformity and a good adaptability to cast polymerization molding and also having a high refractive index, and also relates to a diethylenically unsaturated monomer which gives this optical resin.
(2) Description of the Related Art
Recently, highly transparent plastic materials are used as optical articles such as lenses, optical fibers and optical disc substrates instead of inorganic glass materials while utilizing excellent characteristics of these plastic materials such as a light weight, an easy moldability and a high safety.
However, in case of, for example, polymethyl methacrylate or polydiethyleneglycol bisallyl carbonate, the refractive index .eta..sub.D is as low as 1.49 to 1.50, and when the material is used for lenses and the like, requirements of further reduction of the weight and further compaction are not sufficiently satisfied.
In case of, for example, polystyrene and polycarbonate, the refractive index .eta..sub.D is about 1.58 to about 1.59 and is relatively high, but since these materials are thermoplastic resins, the heat resistance is poor and optical strain is readily caused by the birefringence at the molding step. Moreover, these materials are defective in that the solvent resistance and scratch resistance are poor.
Use of a crosslinkable monomer having a high refractive index is recently proposed for providing a material having a high refractive index and overcoming the foregoing defects. For example, use of polyfunctional monomers having an aromatic ring substituted with a halogen atom, such as a tetrabromobisphenol A skeleton (Japanese Unexamined Patent Publication No. 57-147505, Japanese Unexamined Patent Publication No. 58-18602, Japanese Unexamined Patent Publication No. 60-51706, Japanese Unexamined Patent Publication No. 61-64716 and Japanese Unexamined Patent Publication No. 61-28901) and a tetrabromophthalic acid skeleton (Japanese Unexamined Patent Publication No. 60-166307, Japanese Unexamined Patent Publication No. 60-173006 and Japanese Unexamined Patent Publication No. 60-137912), and monomers having a styrene group introduced as the functional group in an alkylene sulfide skeleton (Japanese Unexamined Patent Publication No. 59-164501) as main components or comonomer components has been proposed.
However, most of these polyfunctional monomers having a high refractive index are solid at room temperature, and therefore, cast polymerization molding of these monomers is difficult and a reactive diluent such as styrene is used in combination while admitting that this results in reduction of the refractive index. In case of the monomer having a styrene group introduced in an alkylene sulfide skeleton, if the chain length of the alkylene sulfide skeleton is short, troubles such as cracking at the cast polymerization molding and breaking at the release from a mold are often caused, and the impact resistance is degraded. These defects are due to the fact that the distance between crosslinking sites is unduly short, and these defects are eliminated by increasing the length of the alkylene sulfide skeleton, but this results in reduction of the refractive index. Moreover, the halogen-containing monomer is defective in that the specific gravity is large.